The postnatal development of adrenergic receptor mechanisms is being studied in newborn piglets of various ages. Cardiovascular responses to adrenergic receptor-blocking agents, and to adrenergic stimuli before and after blockade of receptor systems, are recorded in intact animals and isolated heart preparations. Adrenergic stimuli include catecholamine injections, efferent sympathetic nerve stimulation, and peripheral baroreceptor reflex activation. The cardiovascular responses studied include heart rate, arterial and ventricular pressures and regional blood flows (femoral, renal, cephalic and coronary). The latter are measured by electromagnetic flowmeters. After a control period of one or more types of adrenergic stimulation, the effective blocking dose of a particular agent (phentolamine, phenoxybenzamine, propranolol) is determined; then the same types of adrenergic stimulation are repeated. The existence of age-dependence for minimal effective receptor-blocking dose, and for any difference in degree of cardiovascular response before and after receptor blockade, will be established statistically.